There is illustrated herein in embodiments, methods and systems for adjusting image quality or image consistency in multiple printing or marking engine systems. Embodiments will be described in detail with reference to electrophotographic or xerographic print engines. However, it is to be appreciated that embodiments associated with other marking or rendering technologies are contemplated.
It is desirable, in the use of any system, for an output of the system to match some target or desired output. For instance, in image rendering or printing systems, it is desirable that a rendered, or printed, image closely match, or have similar aspects or characteristics to, a desired target or input image. However, many factors, such as temperature, humidity, ink or toner age, and/or component wear, tend to move the output of a rendering or printing system away from the ideal or target output. For example, in xerographic marking engines, system component tolerances and drifts, as well as environmental disturbances, may tend to move an engine response curve (ERC) away from an ideal, desired or target engine response and toward an engine response that yields images that are lighter or darker than desired.
To combat these tendencies, rendering systems or marking engines are designed with closed loop controls that operate to drive the engine response curve of a marking engine back toward the ideal or target response.
For example, optical sensors are used to sense the reflectance of multiple intra-image or intra-document halftone test patches. The resulting reflectance values are compared to stored reference or target values. Error values, resulting from these comparisons are used to adjust xerographic process actuators. This process is repeated until the errors are minimized, and performed on an ongoing basis in order to prevent or limit engine response curve variation.
Additional control loops are also employed. For instance, electrostatic volt meters are used to measure a charge (or a voltage associated with the charge) placed on a photoconductive belt or drum. The level of charge placed on the photoconductor is a factor in the amount of toner attracted to the photoconductor during a development process. A xerographic actuator, such as a corotron or scorotron wire voltage or a scorotron grid voltage, is controlled so that a measurement received from the electrostatic volt meter (ESV) is driven toward a voltage target or setpoint. The setpoint may be changed to darken or lighten an image.
Toner concentration (TC) sensors can sense, for example, magnetic reluctance associated with magnetic carrier particles, or a developer mixture, in a developer housing. When the toner concentration is high, the average spacing between the magnetic carrier beads is greater and the reluctance signal is lower. As the TC sensor magnetic reluctance signal changes, from a toner concentration/magnetic reluctance setpoint, the rate at which fresh toner is dispensed into the developer housing is changed. The amount of toner transferred to the photoconductor can be a function of the toner concentration in the developer housing. Therefore, changing the toner concentration in the developer housing may affect the lightness or darkness of a rendered or printed image. Therefore, the toner concentration/magnetic reluctance setpoint may be adjusted to lighten or darken an engine response curve or drive an engine response curve toward an ideal or desired position.
Using these sensors and the associated control loops is an effective approach to stabilizing and/or controlling engine response curves. However, these sensors and associated controls are associated with costs and physical space requirements. There is a desire to reduce both the cost and size of marking engines. Therefore, there is a desire for systems and methods that maintain image quality, while eliminating the need for some or all of these sensors and associated control loops.
Some marking engine designs use feed-forward adjustment of process actuators based on lookup tables instead of run time density control. For example, temperature, relative humidity, print count, paper size and other parameters are used to generate and index into one or more lookup tables. The lookup tables provide setpoints for one or more xerographic actuators. Such systems also provide effective engine response curve stabilization. However, over time, due to system wear and other sources of drift, the setpoints stored in the tables can become outdated or inappropriate. Such systems would benefit from a simple and inexpensive means for recalibration, trimming or fine tuning.
Additionally, in order to provide increased production speed, document processing systems that include a plurality of marking engines have been developed. For example, the following co-pending applications, assigned, or under a duty to be assigned, to the same assignee as the present application, and which are hereby incorporated herein by reference for all they disclose, are related to aspects of multi-marking engine systems including but not limited to issues of sheet transportation and engine calibration and consistency using internal sensors: U.S. patent application Ser. No. 10/924,458 by Lofthus, et al. filed Aug. 23, 2004 and entitled PRINT SEQUENCE SCHEDULING FOR RELIABILITY; U.S. patent application Ser. No. 10/917,676 by Lofthus, et al. filed Aug. 13, 2004 and entitled MULTIPLE OBJECT SOURCES CONTROLLED AND/OR SELECTED BASED ON A COMMON SENSOR; U.S. patent application Ser. No. 10/761,522 by Mandel, et al. filed Jan. 21, 2004 and entitled HIGH PRINT RATE MERGING AND FINISHING SYSTEM FOR PARALLEL PRINTING; and U.S. patent application Ser. No. 10/917,768 by Lofthus filed Aug. 13, 2004 and entitled PARALLEL PRINTING ARCHITECTURE CONSISTING OF CONTAINERIZED IMAGE MARKING ENGINES AND MEDIA FEEDER MODULES.
In such systems, the importance of engine response control or stabilization is amplified. Subtle changes that would go unnoticed in the output of a single marking engine can be highlighted in the output of a multi-engine image rendering or marking system. For example, the facing pages of an opened booklet rendered or printed by a multi-engine printing system can be rendered by different devices. For instance, the left hand page in an open booklet may be rendered by a first print engine while the right-hand page is rendered by a second print engine. The first print engine may be rendering images in a manner just slightly darker than the ideal and well within a single engine tolerance. The second print engine may be rendering images in a manner just slightly lighter than the ideal and also within the single engine tolerance. While an observer might not ever notice the subtle variations when reviewing the output of either engine alone, when their output is compiled and displayed in the facing pages of a booklet the variation may become noticeable and be perceived by a printing services' customer as an issue of quality.
The following cited Patents are also hereby incorporated herein by reference for all they disclose.
U.S. Pat. No. 4,710,785, which issued Dec. 1, 1987 to Mills, entitled PROCESS CONTROL FOR ELECTROSTATIC MACHINE, discusses an electrostatic machine having at least one adjustable process control parameter. The machine receives and stores electrical image information of an original. A reproduction of the original is created using the received electrical image information signal, and a second electrical image information signal is in turn created from the reproduction. The second electrical image information signal is compared with the first electrical image information signal to produce an error signal representative of differences therebetween. The process control parameter is adjusted in response to the error signal to minimize said differences.
U.S. Pat. No. 5,510,896, which issued Apr. 23, 1996 to Wafler, entitled AUTOMATIC COPY QUALITY CORRECTION AND CALIBRATION, discloses a digital copier that includes an automatic copy quality correction and calibration method that corrects a first component of the copier using a known test original before attempting to correct other components that may be affected by the first component. Preferably, a scanner subsystem is first calibrated by scanning a known original and electronically comparing the scanned digital image with a stored digital image of the original. A hard copy of a known test image is then printed by a printer subsystem and the calibrated scanner subsystem scans the hard copy. The scanned digital image is electronically compared with the test image and the printer subsystem is calibrated based on the comparison.
U.S. Pat. No. 5,884,118, which issued Mar. 16, 1999 to Mestha, enitled PRINTER HAVING PRINT OUTPUT LINKED TO SCANNER INPUT FOR AUTOMATIC IMAGE ADJUSTMENT, discloses an imaging machine having operating components including an input scanner for providing images on copy sheets and a copy sheet path connected to the input scanner. The imaging machine is calibrated by providing an image on a first copy sheet and automatically conveying the first copy sheet to the input scanner by way of the copy path. The image on the first copy sheet is scanned and provides the image on a second copy sheet. The image on the second copy sheet is sensed and compared to a reference image to calibrate the imaging machine. The calibration sequence is automatically initiated via control data stored in memory.
U.S. Pat. No. 6,418,281, which issued Jul. 9, 2002 to Ohki, entitled IMAGE PROCESSING APPARATUS HAVING CALIBRATION FOR IMAGE EXPOSURE OUTPUT, discusses a method wherein a first calibration operation is preformed in which a predetermined grayscale pattern is formed on a recording paper and this pattern is read by a reading device to produce a LUT for controlling the laser output in accordance with the image signal (gamma correction). A second calibration operation is performed after the first calibration operation wherein a patch is formed on an image carrier by the laser output controlled by the above LUT, its density is detected by a detector and a correction LUT is generated in accordance with the detected density.
However, these Patents are not concerned with methods for improving or achieving image consistency between or among a plurality of marking engines.
For the foregoing reasons, there is a desire for methods and systems for calibrating, trimming, adjusting or fine tuning marking engine controls or setpoints, while eliminating or reducing the need for, or accuracy requirements of, at least some internal marking engine sensors.